The dissolution of silicon dioxide (SiO.sub.2) in a hydrofluoric acid (HF) solution is a fundamental step in the fabrication of integrated circuits (IC). The overall chemical reaction involved is usually understood as: EQU SiO.sub.2 +6HF.fwdarw.2H.sup.+ +SiF.sub.6.sup.= +2H.sub.2 O (1)
Etch rates and etch profiles are usually controlled by buffering and/or diluting the HF solution and by doping or altering the oxide layer via thermal diffusions or ion implantation. Based on conventional knowledge behind these techniques, etching of an IC structure such as that shown in FIG. 1 occurs isotropically, with only a modest decrease in the etch rate of the acceptor doped material 10 found over the p.sup.+ region 20 when compared to the donor doped material 30 found over the n.sup.+ region 40. A slight overetch on the p.sup.+ side 10 of the bore 50 through the photoresist 60 is sometimes necessary due to the added oxide thickness in region 30, but this is a minor difficulty, easily accomplished in order to clear both areas 10 and 30 of oxide.